Right now Earth is surrounded by over 13,500 chunks of space rock, and if one of them bumps into us, the damage could be enough to flatten cities, wipe countries off the map, or even cause global extinctions.
"We are in a shooting gallery," Nahum Melamed, a project manager with the Aerospace Corp., recently said during an event for the American Institute of Aeronautics and Astronautics in California.
Now NASA is getting serious about the potential threat by creating the Planetary Defense Coordination Office (PDCO). The office will track the potentially dangerous objects, warn us of any close passes, and continue working with the Federal Emergency Management Agency (FEMA) and the US Department of Defense to figure out what we'll do if one ever heads directly for Earth.
Right now NASA keeps tabs on these threats through its near-Earth object (NEO) observation program. If an asteroid, comet, or other celestial rock passes within 1.3 astronomical units of the sun - 1 au is 93 million miles, or the distance from the Earth to the sun - then NASA tags it as an NEO. If it comes within 0.05 au of Earth - a little more than 4.5 million miles - then it's labeled as a potentially hazardous NEO.
NASA has already recorded over 13,500 NEOs, and over 1,600 are potentially hazardous.
You can tell why Melamed says we're in a shooting gallery in the map of near-Earth objects by the Armagh Observatory below. These NEOs are orbiting the sun, but they move faster than Earth and their orbits are less stable, so they could potentially come careening in our direction.
At the center of the yellow circled area is Earth - the yellow dot in the center is the sun. All the green dots represent asteroids at a safe distance, while red and yellow dots represent a ring of potentially dangerous asteroids:
Luckily, all of the NEOs we've found have less than a 0.01% chance of hitting Earth within the next century, according to NASA, and the next potentially dangerous - though still not at all likely - encounter won't happen until 2027.
But sometimes NASA misses an NEO, like the one that unexpectedly flew past Earth on Halloween in 2015. And even though there's a low likelihood that Earth will get hit, asteroids come hurtling toward Earth at hundreds of miles per hour, and the potential damage is not something to take lightly. Remember what happened to the dinosaurs?
We know that these impacts have happened regularly, if not often, throughout Earth's history because there's evidence buried in rock layers on the planet. But impacts are incredibly difficult to predict. They don't happen in any kind of predictable pattern or strike in a predictable spot. And if one ever heads right for us, we don't have a plan.
"While there are no known impact threats at this time, the 2013 Chelyabinsk super-fireball and the recent 'Halloween Asteroid' close approach remind us of why we need to remain vigilant and keep our eyes to the sky," John Grunsfeld said in a NASA press release.
Most importantly, the newly formed PDCO will be working on a way to redirect a dangerous asteroid should one ever head for Earth.
One potential plan is NASA's Asteroid Redirect Mission concept. This plan would use the gravity of another object to tug the incoming NEO slightly off its original orbital path. Other possible plans that PDCO will work on include a spacecraft that will fire at an NEO to deflect it.
If all else fails, then NASA will be in direct contact with FEMA to get us as prepared as possible.
"Even if intervention is not possible, NASA would provide expert input to FEMA about impact timing, location and effects to inform emergency response operations," according to a NASA post. "In turn, FEMA would handle the preparations and response planning related to the consequences of atmospheric entry or impact to U.S. communities."